Nitrate reductase (NR) and nitrite reductase (NiR) play important roles in nitrate assimilation in plants. Previous studies have indicated that NR and NiR in eelgrass may contribute to its NaCl tolerance. This study investigated the expression characteristics and the biological functions of NR and NiR in eelgrass (Zostera marina), named as ZmNR and ZmNiR, were cloned, characterised and overexpressed in both bacteria and tobacco. The open reading frames of ZmNR and ZmNiR contain 2628 and 1773 nucleotides that encode 875 and 590 amino acids respectively. Amino acid sequence alignment indicated that the purported ZmNR and ZmNiR proteins presented low homology with other plant NR and NiR sequences. Real-time quantitative PCR revealed that the expression of ZmNR and ZmNiR was supressed when exposed to low salinity and induced by high salinity. Further physiological analyses demonstrated that blocking nitrate assimilation by adding Na2WO4 in eelgrass reduced its tolerance to NaCl stress. The heterologous expression of the ZmNR and ZmNiR genes in Escherichia coli and Nicotiana benthamiana could confer tolerance to NaCl stress. Physiological and growth analyses suggested that ZmNR and ZmNiR in plants could resist NaCl stress by regulating various physiological pathways and biochemical processes triggered by nitric oxide (NO). Taken together, these results suggested that NR-dependent NO synthesis may play an important role in NaCl tolerance in eelgrass.
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